The invention relates to a process and apparatus for stopping and restarting a machine for pneumatic false-twist spinning of the type having a plurality of spinning units which each have at least one drafting frame, one pneumatic false-twisting nozzle, one withdrawal device and one wind-up device. A main motor is provided for driving the drafting frames, the withdrawal devices and the wind-up devices. A blower motor for a blower is connected with the false-twist nozzles via at least one solenoid valve. The main motor is switched off and on for the stopping and restarting, and the blower motor and the solenoid valves are switched as a function of the main motor.
In pneumatic false-twist spinning, the strength of the spun yarn is caused exclusively via the air nozzles (the pneumatic false-twisting nozzles). This means that, when a false-twist spinning machine is stopped and restarted, yarn breakages will occur immediately if the air nozzles are no longer acted upon by compressed air and the drafting frames and the withdrawal devices are still running. For stopping a machine for pneumatic false-twist spinning without any yarn breakages, it is known (DE-25 33 655 C2) corresponding to U.S. Pat. No. 3,992,865 to Tuchida et al., to switch time function elements together with the switching of the main motor. Via these time function elements, the blower motor and the solenoid valves are switched off such that the main motor is stopped before the compressed-air supply to the air nozzles is switched off. During the restarting, the sequence is reversed; i.e., first the blower motor and the solenoid valves are switched on, after which, after a given time delay, the main motor is restarted. In this case, the time function elements are set to a time that, on the basis of experiments, is required until the main motor is stopped completely or, vice versa, has regained its normal speed. These types of experimental values must be determined and set at each machine because, on the one hand, the slow-down behavior and the starting behavior depend on the machine, i.e., the number of spinning points or units and the respectively set operating speed. On the other hand, deviating requirements with respect to strength and also with respect to the danger of yarn breakage exist for every yarn size, so that readjustments are required in each case. In this case, it must be taken into account that the compressed-air supply must also not remain switched on for a considerable period of time after the stoppage of the main motor because otherwise there is the danger that the yarns are torn as a result of overtwisting.
An object of the invention is to provide a process and apparatus of the initially mentioned type by means of which the stopping and the restarting may be carried out with values that were set once, irrespective of the operating conditions of the machine and of the spun yarn type.
This object is achieved in that the start-up time and the slow-down time of the main motor are controlled to preselected values.
In a further development of preferred embodiments of the invention, it is provided that the rotational speed of the main motor is measured during the stopping and during the restarting, and in that the solenoid valve and the blower motor, during the stopping, are not switched off before the rotational speed of the main motor has fallen to a preselected value, and in that the solenoid valve and the blower motor, during the restarting, are not switched on before the rotational speed of the main motor has reached a preselected rotational speed. Thus, a process for the stopping and restarting is provided that is largely independent of the design or the method of operation of the respective false-twist spinning machine because the switching always takes place at given rotational speeds.
In a further development of preferred embodiments of the invention, it is provided that, for triggering the switching of the solenoid valve and of the blower motor, during the stopping and/or during the restarting, the reaching and/or the leaving of the zero rotational speed is preselected. As a result, the slow-down behavior and also the start-up behavior of the main motor is automatically also taken into account, while, nevertheless, the switching-on and/or the switching-off of the compressed-air supply always takes place at an appropriate point in time. The compressed-air supply remains fully switched on until the main motor has come to a stop. As a result, the yarn is twisted a little too much during the stopping phase. However, during the switching-on of the main motor, the air supply is switched on again as soon as the main motor is set in motion. Since a certain time span is required until the complete air pressure is available again at all air nozzles or pneumatic false-twisting nozzles, the yarns, during the start-up, receive slightly too little twist. The overtwisting during the stopping is therefore compensated by the slightly reduced twisting during the restarting.
In a further development of preferred embodiments of the invention, it is provided that the solenoid valve, during the stopping, connects the air nozzles with the atmosphere. As a result, it is ensured that, immediately with the actuating of the solenoid valve, the compressed-air supply is interrupted, without the switch-off time or the slow-down time of the blower and of the blower motor having any influence on this interruption.
In a further development of preferred embodiments of the invention, it is provided that the solenoid valve, during the stopping, connects the air nozzles, via a preferably adjustable throttling device, with the atmosphere. By means of the preferably adjustable throttling device, the switching-off of the compressed-air supply can be influenced further.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.